SIADH: mechanism and diagnostic criteria
In SIADH (syndrome of inappropriate antidiuretic hormone secretion), ADH (the hormone that makes the kidney retain water) is released despite dilute blood, so the urine is inappropriately concentrated.
Remember
SIADH is dilute blood with urine that is not maximally dilute (urine osmolality, how concentrated the urine is, above 100 mOsm/kg) and urine sodium above 30 mmol/L on a normal salt and water intake, in a euvolaemic patient (neither dry nor swollen), diagnosed only once thyroid, adrenal and renal disease and diuretic use are excluded.
Dilute blood should switch ADH off, so the kidney makes dilute urine. In SIADH, ADH is released although the blood is dilute. ADH keeps acting on the collecting duct (the final part of the kidney's tubules), where it places aquaporin-2 channels (pores that let water cross) in the cell membrane. Water is pulled back into the blood, so the kidney cannot get rid of free water (water without dissolved particles).
The retained water dilutes the blood. Plasma osmolality (how concentrated the blood is) is low, below 275 mOsm/kg, so the hyponatraemia (a low blood sodium) is genuinely hypotonic. The urine is inappropriately concentrated, with an osmolality above 100 mOsm/kg.
The urine sodium is raised, usually above 30 mmol/L on a normal salt and water intake, because the patient is euvolaemic (has a normal circulating volume, neither dry nor oedematous), so the kidney has no reason to save sodium.
SIADH is a diagnosis of exclusion. Severe hypothyroidism (an underactive thyroid) and adrenal insufficiency (too little cortisol) can look like it, so thyroid and adrenal function must be normal. Renal function must also be normal, and there must be no recent diuretic use. Once SIADH is established, the next job is to look for what is driving the ADH release, such as lung disease, brain disease or drugs.
Key facts
- Blood
- plasma osmolality is below 275 mOsm/kg, so the hyponatraemia is hypotonic.
- Urine
- osmolality is above 100 mOsm/kg and sodium is above 30 mmol/L on a normal salt and water intake.
- Volume status
- clinically euvolaemic, with no oedema and no dehydration.
- Exclusions
- normal renal, adrenal and thyroid function, and no recent diuretic use.
- Criteria
- the usual diagnostic criteria are the modified Bartter and Schwartz criteria.
- Trigger
- an SSRI (a type of antidepressant) such as sertraline can trigger SIADH.
Exam traps
- Primary polydipsia (drinking excessive water) is a euvolaemic cause of hypotonic hyponatraemia, but it does not explain concentrated urine, because ADH is then switched off and the urine osmolality is below 100 mOsm/kg.
- Very high blood glucose pulls water out of cells and dilutes the sodium, but it raises plasma osmolality, so it gives hypertonic hyponatraemia rather than the hypotonic picture of SIADH.
- Adrenal insufficiency (too little cortisol) can give the same concentrated urine and raised urine sodium as SIADH, because losing cortisol removes the normal suppression of ADH, so it has to be excluded before SIADH is diagnosed.
Read the full notes
Tested in 9 SimpleMed+ questions
Practice it in Urinary System, with the answer and a full explanation after every question.
Practice these questions